Physical properties of food are characteristic values important to controlling food qualities in processing, distribution and consumption. In particular, if a viscosity can be easily measured, it is possible to know not only processing aptitudes for cooking, filling and so on but also textures and easiness in handling, as well as comparison with another food can be facilitated.
There are various types of viscosity measuring apparatuses, and the types are roughly classified into a rotation type and a translation type.
The rotation-type viscosity measuring apparatus is advantageous in providing a simple measurement at low cost, and is suited for measuring a uniform sample having a low viscosity. However, when a sample having a high viscosity such as a gel is measured by the rotation-type viscosity measuring apparatus, an internal structure of the sample varies because of a “shear deformation” or vibrations given thereto until a measurement value becomes stable. Thus, there is a problem in that a viscosity of the sample is measured to be lower than an actual viscosity.
On the other hand, the translation-type viscosity measuring apparatus is advantageous in having a simple apparatus structure, without any rotating and driving unit. There are a translation-type viscosity measuring apparatus of a parallel plate type and a translation type viscosity measuring apparatus of a concentric cylinder type. JP3446117B discloses a viscosity measuring method using a viscosity measuring apparatus of a concentric cylinder type.
FIGS. 25 and 26 are schematic views for explaining a viscosity measuring method of JP3446117B. In the viscosity measuring method of JP3446117B, a plunger having an outer radius Ri is firstly immersed into a sample contained in a cylindrical container having an inner radius R0, coaxially with the cylindrical container, by an initial depth L0 (see FIG. 25(a)). Then, the plunger is further immersed into the sample coaxially with the cylindrical container at a relative movement velocity vp. In the further immersing operation, a force (stress) applied to the plunger from the sample is measured with a passage of time (see FIG. 25(b)), so as to obtain a force-time curve. Then, based on the force-time curve, an initial value Fv0 of the force at a moment when the further immersing operation was started is obtained. To be specific, an approximate curve L is obtained for a plurality of measurement points in the vicinity of a given time ts in the force-time curve, by the software NRCC Visco-PRO manufactured by Sun Scientific Co., Ltd., so as to obtain a value Fv0 of an intersection point between the approximate curve L and a time t=0 (see FIG. 26). After that, a viscosity μ of the sample is calculated, based on the obtained initial value Fv0 and the following expression (26).
                    μ        =                              -                          1                              2                ⁢                                  πα                  2                                ⁢                                  v                  p                                                              ⁢                      (                                          F                                  V                  ⁢                                                                          ⁢                  0                                                            L                0                                      )                                              (        26        )            in which
      κ    =                  R        i                    R        0                        α      2        =                  1        +                  κ          2                                                  (                          1              +                              κ                2                                      )                    ⁢          ln          ⁢                                          ⁢          κ                +                  (                      1            -                          κ              2                                )                    
By the way, “non-Newtonian fluid” is a fluid whose viscosity is dependent on a “shear rate”. A viscosity of non-Newtonian fluid is represented as an apparent viscosity obtained by dividing a “shear stress” by a “shear rate”.
According to JP3446117B, in a measurement of a non-Newtonian fluid, a viscosity μ, which is calculated as described above, is referred to as an apparent viscosity μa.